Wood is pulped to produce papermaking fibers by any of several different processes, each of which is designed to take maximum advantage of the characteristics of the particular type of wood fibers obtained in the paper ultimately produced. Paper for packaging and container uses, for example, must have a high degree of strength and integrity. For this reason softwood, with its characteristically long and inherently strong fibers, is chemically pulped to yield a furnish having the properties needed in papers of this type.
Paper used as newsprint or in other printing applications need not have the same degree of strength as packaging and container papers. Consequently, the mechanical pulping processes used to make furnishes for newsprint and like papers, including groundwood, thermomechanical (TMP) and chemical thermomechanical (CTMP) pulp processes, are designed to give higher yields of fibers of shorter average lengths, and more fines, than are obtained using typical chemical pulping processes. And while papermaking fibers obtained by mechanical pulping have certain properties that are in general undesirable --low brightness, color reversion with time and high resin contents -- this is usually tolerated because of the cost savings achievable using these high yield pulping processes.
Pulps or furnishes used in continuous processes for making paper generally consist of fibers, fines, fillers and chemicals suspended or dissolved in water. It is desirable to retain the fines in the paper produced, not only for purposes of pollution control but also to insure the greatest possible utilization of the wood being pulped and the other substances added and hence increase profitability. Such fines suspensions may contain pitch colloids not retained initially in the fiber mat, cellulose fines and hemicellulose, recirculated talc, kaolin or other fillers and pigments, and water soluble materials such as alum.
Fines create a number of manufacturing problems, chiefly due to the difficulty of separating water used to make paper from the papermaking fibers while retaining fines with the fibers. Fines not retained with the fibers recirculate with the process water, and increasingly large concentrations of fines in the process water adversely affect drainage and paper formation. Reduced drainage can slow papermaking speed and increase the cost of making the paper. Poor paper formation reduces the quality of the manufactured sheet, causes production losses, or both, and thus can also increase manufacturing cost. The inability to incorporate fines in the paper and consequently the waste of this material, and particularly the paper fines portion of the pulp product, will further increase costs.
Fines retention does not necessarily mean retention of all the fines present. Prior art processes typically used to make the more expensive fine papers focus on the retention of fillers and pigments in the paper while maintaining wet strength and adequate filler loading. Newsprint is relatively inexpensive in comparison to finer grades of paper. Additives such as fillers and retention aids are not normally used in newsprint, since to do so would greatly increase its cost.
As indicated above, papermaking pulps are usually made in one of three ways: by chemical, semichemical, or mechanical pulping processes. In preparing chemical pulps, e.g., by the Kraft or sulfate and sulfite processes, cellulose fibers constituting the pulp are separated from other wood components by chemical reagents. Kraft and bleached pulps contain minor amounts of pitch, present as saponified pitch or pitch soap. Semichemical processes, usually used on deciduous wood species, involve a mild chemical action followed by mechanical attrition. Mechanical pulps are prepared by grinding whole logs, without prior debarking, and thus the pulp obtained contains all of the constituents of the log, including pitch.
Mechanical pulps such as groundwood and thermomechanical pulps commonly used in newsprint mills, sulfite mills and like operations are typically prepared from nondeciduous wood species such as Sitka Spruce, Norway Pine and the like. The large amounts of pitch found in mechanical pulps cause severe pitch problems in papermaking facilities using them, and yet despite extensive studies no completely satisfactory method for pitch control in processes used to make newsprint has been developed prior to this invention. Groundwood pulp contains large amounts of completely non-saponified pitch in the form of colloidal dispersions whose particles generally range in size from about 0.2 to about 2 .mu.m and normally have an electronegativity of from about -10 to about -35 mV. Thermomechanical pulps contain the same types of pitch found in groundwood pulps but generally in greater amounts.
Resins carried over from wood or bark pulped to make papermaking furnishes are complex mixtures of organic compounds which, because they are insoluble in water, will deposit on the fibers in the furnish and on the papermaking equipment. Aggregations of these resinous materials of larger than colloidal dimensions will interfere with the manufacture of high quality paper. Such resinous materials will, for simplicity's sake, be referred to collectively hereinafter as pitch or pitch colloids.
Pitch colloids found in pulp generally are oleophilic, water insoluble, low molecular weight, relatively non-polar coagulable resins which are usually made up of at least several of the following components: fatty acids and resin acids having ionizable hydrophilic groups, esters of these acids, sterols, diglycerides, triglycerides, terpenes, waxes and various alcohols, hydrocarbons, and neutral compounds associated with these resins. The degree of fluidity of these resins is governed by the ratio of fatty acids to resin acids, the age of the wood, and the degree of oxidation or polymerization of the pitch. These factors determine the resins' deposition tendencies. In aqueous wood pulp slurries such resins are present on the surfaces of the fibers in the form of thin patches and droplets, inside parenchyma cells, as soluble soaps, and in the form of colloidal droplets dispersed in the process liquid among the fibers.
These resin or pitch particles agglomerate to give sticky films or pitch balls, which in turn can give rise to spotting in the final paper product, wire spots, localized sticky spots on rolls, holes in the paper sheet, poor paper formation, felt plugging or sticking on dryer and calender rolls.
To control pitch effectively it is not necessary to remove all of the pitch from the pulp. Different types of pulp mills and paper mills have different tolerance levels for pitch particles which must be exceeded before pitch-caused problems occur. What is necessary, however, is that the pitch be dispersed as tiny, non-harmful aggregated or agglomerated particles and retained in this fine particulate form rather than as larger, harmful globs in the paper sheet. Simply flocculating pitch will not give good pitch control; the pitch must be made to attach to either the cellulosic fibers or to any particulate matter used as a filler in the paper sheet, or preferably to both. Pitch retained in large flocs causes breaks and undesirable dark specks in the finished paper sheet, while pitch not retained but in small flocs tends to accumulate in the papermaking facility's white water system.
Effective pitch control, particularly in paper mills which produce paper from mechanical pulps, e.g., groundwood pulp mills which produce newsprint, has proven to be complex and difficult to accomplish even when attempted. Typically, attempts to control pitch during papermaking involve either physically removing pitch particles from the papermaking system or in some fashion decreasing their deposition on the papermaking equipment, e.g., by attempting to finely disperse the pitch and remove it in the paper, a procedure which up to now has not been entirely successful.
Pitch particles have been physically removed from papermaking systems by methods such as pulp fractionation, which involves removing a substantial portion of the colloidal pitch present together with a portion of the pulp fines. Such methods are prohibitively expensive, since they significantly increase the amount of pulp needed to make a given quantity of paper and pulp fines removed in the process are lost.
Attempts to decrease pitch deposition by dispersing or precipating the pitch have typically involved the use of alum, talc, chemical dispersing agents, retention aids, or mixtures of such materials. The idea behind using such substances is to prevent or reverse pitch agglomeration and the formation of larger pitch particles which can collect on papermill machinery and also appear as dirt specks or globs in the paper.
Canadian Patents Nos. 454,036, issued Jan. 11, 1949 to Carpenter et al and 565,955, issued Nov. 11, 1958 to Craig et al, teach the use of alum and sodium aluminate, respectively, for this purpose. While both of these substances can provide noticeable reductions in the amount of pitch present during papermaking, neither by itself will overcome the pitch problem entirely.
Typically, alum-based pitch controlled systems in which alum is used alone or together with a base such as sodium hydroxide attempt to control pitch deposition by first dispersing the pitch, if it is not already sufficiently dispersed, and then flocculating it. Microscopic examination of this floc shows very little of the pitch actually attached to the papermaking fibers. Instead, the pitch is largely attached to itself as agglomerates consisting of many pitch particles clustered together. These clusters are small enough to pass through the wire of a newsprint mill and get into the whitewater system, resulting in pitch buildup. Additionally, alum is extremely corrosive, necessitating rigorous pH control, in high concentration causes drainage and retention aid losses, and in some papers may also contribute to a loss of brightness, or yellowing, with age accompanied by loss of strength.
Talc per se, such as Mistron Vapor talc produced by Cyprus Industrial Minerals Company, has been used to control pitch in pulp and paper mills, usually Kraft or sulfite mills operating at a high enough pH to cause any pitch present to be partially saponified. The dispersed pitch particles are adsorbed on the faces of the talc particles and then retained in the paper. Attempts have been made to increase pitch adsorption by increasing the talc's surface area. However, too finely ground talc loses crystallinity and all ability to adsorb pitch; talc's pitch adsorption efficiency appears to peak at a surface area of about 15 square meters per gram.
U.S. Pat. No. 4,305,781, issued Dec. 15, 1981 to Langley et al and related Canadian Patent No. 1,168,404, issued June 5, 1984, teach the use of bentonite clay as a filler, together with a water soluble, high molecular weight, substantially nonionic polymer, such as a polyacrylamide, a poly(dialkylaminoalkylacrylate) or polyethylene oxides (PEO), in papermaking, and state that this system results in a reduction in solvent extractable resinous pitch content in the mill's white water system.
European pulp and paper mills are known to use a dual filler retention system in which a naturally occurring cationic bentonite is introduced into the headbox after the last point of high shear, followed by the addition of a high molecular weight anionic polymeric flocculant. Bentonite mined from specific deposits has been demonstrated to exhibit a moderate degree of pitch control in addition to filler retention properties. Attempts to replace these specific bentonites with bentonites from other deposits, however, or with mica or talc, have proven unsuccessful from both a pitch control and a filler retention standpoint.
Modified bentonite clays have also been used as cationic scavengers in mill process water to neutralize fiber fines and other anionic trash.
Chemical dispersants, generally anionic or nonionic in nature, have been used to maintain pitch colloids in a dispersed state in papermaking systems -- in the pulp, stock, furnish and whitewater -- to prevent pitch deposition within the system. These dispersants function by charge or steric repulsion phenomena. Anionic dispersants, for example, impart a still higher electronegative charge to pitch colloid particles, keeping them from touching and flocculating. Some dispersants may also tend to soften and dissolve those pitch deposits already formed. However, since chemical dispersants do not attach the pitch to the papermaking fibers, they have proven to be inadequate in solving severe pitch problems, such as those found in groundwood pulp (newsprint) mills. Their use in fact can result in a buildup of pitch in the mill's tightly closed recirculated water system.
Retention aids, usually polymeric in nature, have been used to control pitch to a certain extent by flocculating the dispersed pitch resins. In pulps containing large amounts of pitch, however, such as groundwood pulps, the relatively large quantities of dissolved and dispersed polymer-attracted materials present ("anionic trash" or "pernicious objectionables") consume large quantities of the added retention aid, thus rendering them ineffective. To obtain good flocculation it becomes necessary to add still larger amounts of retention aid, which is at best marginally cost effective. Furthermore, the addition of too much retention aid results in overflocculation, adversely affecting the formation of acceptable paper.
A number of prior art patents and literature articles disclose the use of cationic, anionic and nonionic polymers, as well as mixtures of two or more of such polymers, as retention aids which, in addition to performing their primary function, may also control pitch to some extent.
U.S. Pat. No. 3,141,815, issued to Manely on July 21, 1964 discloses that high molecular weight nonionic PEO can increase fines retention during papermaking in certain mechanical pulps, while U.S. Pat. No. 4,070,236, issued Jan. 24, 1978 to Carrard et al, teaches that mixtures of PEO with certain synthetic phenolic compounds perform the same function.
Pelton et al, "A Survey of Potential Retention Aids for Newsprint Manufacture," Paprican, July 1978, notes that retention aids may reduce pitch problems in newsprint mills by flocculating colloidal pitch onto fibers and fines and subsequently incorporating these pitch-coated particles into the paper sheet. Pelton et al observed a decrease in the concentration of colloidal pitch when 1 lb/ton of PEO was added to the headbox stock. PEO retention aids are, however, of limited applicability, since they are sensitive to shear and thus can be used only at locations in the papermaking process at which relatively low hydrodynamic shear forces are found. PEO is also very sensitive to depolymerization by trace amounts of chlorine and hence cannot be used with chlorine bleached pulps.
U.S. Pat. No. 4,313,790, issued Feb. 2, 1982 to Allen et al, discloses adding a Kraft lignin or modified Kraft lignin and polyethylene oxide to a mechanical pulp papermaking furnish to increase fines retention and decrease pitch deposition. At column 7, lines 42-56, of the Allen et al patent, the addition of the Kraft lignin derivative and/or polyethylene oxide is said to decrease the concentration of colloidally dispersed wood resin particles in the white water from 88.times.10.sup.6 to as low as 5.times.10.sup.6 particles per cm.sup.3.
Very high molecular weight organic polymers such as PEO used as filler retention aids have not proven effective for flocculating bleached, groundwood or thermomechanical pulp. See, for example, Leung et al, "Flocculation of Paper Fines by Polyethyleneoxide," Tappi Journal, Vol. 70, No. 7, pages 115-118, July 1987; C.H. Tay, "Application of Polymeric Flocculant in Newsprint Stock Systems for Fines Retention Improvement," Tappi Journal, Vol. 63, No. 6, pages 63-66, June 1980; Pelton et al, "Novel Dual-Polymer Retention Aids for Newsprint and Groundwood Specialties," Tappi Journal, Vol. 64, No. 11, November 1981.
U.S. Pat. No. 2,795,545, issued June 11, 1957 to Gluesenkamp, discloses adducts prepared by reacting:
". . .inorganic solids that are gel-forming in water and/or that possess ion exchange like properties, e.g., clays [including kaolinite and montmorillonite], . . ." PA0 ". . .cationic polymers. . .[such as homopolymers and copolymers of vinylpyridines, amino alkyl acrylates, amino alkyl methacrylates, and quaternized derivatives thereof, and poly-N-allyl amines]. . ." PA0 ". . .to improve the clay retention in paper making wherein clay is used as a beater additive";
with:
to form "polycations"; see, e.g., column 3, lines 44-61; column 4, lines 9-38; column 5, lines 49-66, column 6, lines 12-39 and column 8, line 32-column 9, line 4. Such "polycations" can then be used:
see, e.g., column 2, lines 44-46 and column 12, lines 30-64.
Polymers or polymer/filler conglomerates for use as filler or pigment retention aids have also been disclosed in the following patents: acrylamide polymers, U.S. Pat. No. 3,052,595; cationic starch-g-poly(N,N'-methylenebisacrylamide-coamine) copolymers, U.S. Pat. No. 4,278,573; cationic starches, U.S. Pat. No. 4,643,804; cationic latexes, U.S. Pat. No. 4,445,970; conglomerates of a high molecular weight substantially anionic or cationic polyacrylamide (molecular weight at least 2,000,000) and a filler such as china clay, talc or titanium dioxide, such conglomerates having a zeta potential between -40 and +40 electron volts (-6.4.times.10.sup.-8 volt to +6.4.times.10.sup.-8 volt), U.S. Pat. No. 4,181,567; and dual polymer systems such as that disclosed in Japanese Laid-Open Patent Application No. 55-163298 to Mitsubishi Paper Mills, Ltd. published Dec. 19, 1980, in which a water soluble or dispersible cationic polymer is first added to a filler and this mixture is then added to a wood pulp slurry containing an anionic polymer, and those disclosed by Britt et al in "Electrophoresis in Paper Stock Suspensions as Measured by Mass Transport Analysis," Tappi Journal, Vol. 57, No. 12, pages 81-84, December 1974, and in "New Methods for Monitoring Retention," Tappi Journal, Vol. 59, No. 2, pages 6770, February 1976, which teaches adsorbing a low-molecular weight cationic polymer, polyethylene imine, onto the solid surfaces of the pulp stock followed by the addition of a long-chain anionic polymer to this suspension, forming a floc that is said to be very resistant to redispersion. None of these patents, patent applications and literature articles, however, disclose control, or even reduction, of pitch when using their polymer or polymer/filler conglomerate retention aids.
In summary, it is evident from the prior art that various polyelectrolytes, by themselves or as polymer/filler conglomerates, have been used for fines retention in making fine papers, and that polyelectrolytes themselves, but not polymer/filler conglomerates, are known to reduce pitch in certain pulps and papermaking furnishes. However, when polyelectrolytes were used in wood furnishes containing more than 10% mechanical pulp they were found to be ineffective both in improving fines retention and in reducing pitch and pitch-related problems. A recent article, Tappi Journal, Vol. 70, No. 10, beginning at page 79 (1987), reviews this situation.
It is generally agreed that polyelectrolytes per se are ineffective in improving fines retention and reducing pitch in wood furnishes containing mechanical pulp for a reason adverted to above, the presence in such furnishes of relatively large quantities of "anionic trash" or "pernicious objectionables". These polymer-consuming substances are not present or present only in inconsequential amounts in pulps prepared by techniques other than mechanical pulping, for example chemical or semichemical methods, because such other pulps are washed to a far greater extent than mechanical pulps.